As market competition becomes increasingly fierce, a performance level of a base station product is a main focus of intertype competition. A power amplifier unit as an important part of a base station, is directly associated with the quality and communication effect of transmitted signals. Currently, a most widely applied mature technique is a Doherty power amplifier technique, and most power amplifier manufacturers have begun to mass-produce and apply Doherty power amplifiers. In the recent years, this technique still occupies a dominant position in terms of cost and mass-production ability. Therefore, how to further reduce the size of the power amplifier based on this power amplifier technique and improve a linear property and efficiency of the power amplifier becomes even more important.
A structure of a traditional Doherty power amplifier is shown by a reference number 102 in FIG. 1, which is composed of more than two power amplifier tubes, and is divided into a main power amplifier tube and an auxiliary power amplifier tube, wherein the main power amplifier tube and the auxiliary power amplifier tube separately work in different working conditions, and improvement of efficiency is achieved through an impedance modulation technique. At a combination output end of a power amplifier circuit, modulation and matching of impedance is generally implemented through an impedance conversion line 103. For example, for a typical Doherty power amplifier circuit, the modulation and matching of the impedance are implemented by the impedance conversion line 103 which is 35 Ohms at a combination output at which two power amplifier tube perform output.
Since a small signal transmitted by a transceiver usually incidentally generates some harmonic signals after amplified by a power amplifier and these harmonic signals may interfere other components, therefore the power amplifier always has a certain requirement on the size of an output harmonic. A general method of reducing the output of harmonic signals is to add a filter at an output end; as shown in FIG. 1, a filter 105 is provided at the combination output end of the Doherty power amplifier circuit.
Additionally, the linear property is usually improved through a Digital Pre-Distortion (DPD) by the Doherty power amplifier circuit, therefore, it needs to sample a power amplifier output signal to perform DPD training, based on which a coupler may be added to the combination output end of the Doherty power amplifier circuit to implement a signal sampling function. As shown in FIG. 1 a coupler 104 is provided in the Doherty power amplifier circuit.
It can be seen from the Doherty power amplifier circuit shown in FIG. 1 that in design of the Doherty power amplifier circuit, a coupler is needed to be provided on a microstrip line of which an output is 50 Ohms to implement a DPD training function or a power control function, and the filter is provided to restrain a harmonic; and these components may occupy a certain space on a Printed Circuit Board (PCB), thus inevitably leading to increase of the size of the microstrip line at a power amplifier output end and increase of a differential loss, and further resulting in decrease of efficiency of the entire power amplifier, increase of a PCB size of the power amplifier circuit, and cost increase of the power amplifier circuit.